Secondary carnitine deficiency is an important feature of 9 inherited disorders of mitochondrial fatty acid oxidation. The goal of this grant is to investigate the mechanism underlying this abnormality and its contribution to the pathophysiology of these disorders. Our hypothesis is that accumulations of fatty acylcarnitines, associated with blocks in fatty acid oxidation, lead to decreased tissue and plasma carnitine concentrations by inhibiting plasma membrane transport of free carnitine. This hypothesis will be tested by studies of 1) the interactions of individual acylcarnitines with the human muscle-kidney carnitine transporter in cultured skin fibroblasts; 2) the effects of genetic fatty acid oxidation defects on the function of this transporter both in affected patients and in their mutant fibroblasts; and 3) the in-vivo changes in tissue acyl-CoA and acylcarnitine concentrations induced by specific blocks in fatty acid beta-oxidation enzymes in experimental animals. In addition, we will investigate how carnitine transport and mitochondrial substrate oxidation are affected by a new genetic defect which blocks the transport of carnitine and acylcarnitines into mitochondria. These experiments will be facilitated by several unique resources, including the availability at The Children's Hospital of Philadelphia of patients with a large number of genetic disorders associated with secondary carnitine deficiency; analogs of Hypoglycin A which irreversibly inhibit specific fatty acyl-CoA dehydrogenase enzymes; and a repository of fibroblast cultures from over 500 patients with fatty acid oxidation disorders.